This invention relates to an improvement of line concentrating stage of a time-division local switching system.
According to a recent trend in telephone switching system electronic switching system is gradually replacing cross bar switching system, for the purpose of space saving and improving the economy and flexibility thereof. It appears that use of time-division switching systems in the speech-path equipment will expand in the future, to further reduce the size and improve the overall network economy. In fact, commercial use of time-division toll switching system has been announced in some countries. However, as far as the local switching system is concerned, the simple use of time-division switching techniques does not fulfill the desired space saving and reduction of cost for the following reasons.
(A) Although use of the time-division multiplex techniques facilitates extreme miniaturization and economization of speech-path switches, it requires a converter per subscriber line (in subscriber line circuits and trunk circuits) for converting voice signals into a time-division multiplexed signal. Such converters cost more than half the entire cost of a time-division speech-path system, so that economization of the converters is an important problem.
(B) The line occupancy of the trunk line is usually high, being 0.5 to 0.7 erlang/line, so that its cost per erlang is low. On the other hand, the subscriber line is used only at a low utilizing rate, e.g., 0.1 erlang/line, so that the cost per erlang is high. Accordingly, it is very difficult to construct an economical local switching system using time-division techniques alone.
(C) As a means for reducing the number of the converters (subscriber line circuit) line concentrator has been proposed, in which space-division switches are provided in the subscriber lines so as to effect the time-division multiplex on a commonly-used subscriber line circuit after traffic concentration (for instance, see KENKYU JITSUYOKA HOKOKU, or Research and Development Report, Vol. 16, No. 11, P. 423, "Speech-path configuration of Switching Equipment DEX-T1"). Such a conventional metallic line concentrator, however, does not reduce the overall cost because it requires a considerable amount of hardware, e.g., subscriber line scanners, metallic crosspoint drivers, signal-transmitting trunks, and so on.